Waveguides are used on a regular basis as a means for facilitating the propagation of electromagnetic waves. Typical waveguides comprise a plurality of conductive walls which define an elongated open section through which electromagnetic waves can propagate. In normal waveguide applications, it is usually not necessary to install a window across the open section of the waveguide. However, in applications where a portion of the waveguide needs to be in a vacuum, or where a section needs to reside in a pressurized environment, a window is installed across the open section of the waveguide to preserve the vacuum or the pressurized environment, and yet allow the electromagnetic waves to propagate through the window along the longitudinal axis of the waveguide.
A typical prior art waveguide window is disclosed in U.S. Pat. No. 4,286,240, wherein the window comprises two dielectric plates 18 (FIG. 1) placed across the open section of the waveguide 10, orthogonal to the longitudinal axis 12. The plates 18 allow electromagnetic waves to propagate through but keep the two halves of the waveguide environmentally separate. The two plates 18 are placed in parallel with each other and are separated by a gap 60. Cooling liquid is pumped through gap 60 to remove from the plates 18 any heat generated by electromagnetic waves propagating through the plates 18. The window described in U.S. Pat. No. 4,286,240 performs adequately in some applications, but for very high power applications in which both the frequency and the power of the electromagnetic waves are high, this prior art window fails to provide satisfactory results.
For high power applications, a large window cross-section is required to prevent electric breakdown and to maintain power dissipation at a sufficiently low level so that heat generated in the window may be dissipated by some cooling means. As the window cross-section is increased, the thickness of the window must also be increased if the window is to withstand the mechanical forces imposed by the coolant flow and the vacuum/pressure environments. Increasing thickness, however, causes the fraction of wave power lost in the window to increase, and this, in turn, requires that the coolant pressure and flow rate be increased to carry away the additional heat. This increased flow rate imposes greater stress on the window, thus, forcing the window to have an even greater thickness. The increased thickness again requires an increased coolant flow rate, and the higher flow rate again requires a greater window thickness. Thus, a cycle of increasing the thickness of the window is created.
The practical effect of this cycle is that the prior art window is capable of handling power levels only up to a certain power limit. For applications where electromagnetic power levels exceed this limit, the prior an window falls to function properly. Typical power limits are 500 KW at 140 GHz, and 800 KW at 110 GHz. Besides being power limited, another drawback of the prior an window is that it is expensive to produce, especially for applications near the power limit. For some applications, the cost of the window is prohibitive.
Another prior art waveguide window is disclosed in U.S. Pat. No. 5,051,715. In a first embodiment shown in FIG. 1 of the patent, this prior art coupling-out window ccmprises a plurality of conductive cooling fins 5a, 5b, 5c disposed among a plurality of strip-like dielectric portions 11a, 11b, 11c, 11d. The cooling fins and strip-like portions are held to each other and to mounting 7 by vacuum-tight laminar thermal contacts. Together, the fins 5a-5b, portions 11a-11d, and mounting 7 form the coupling-out window of the device.
A major disadvantage of this prior art window is that it requires the making of a relatively large number of vacuum-tight joints. Referring to FIG. 1 of the patent, at every point where a dielectric portion 11a-11d comes into contact with either a cooling fin 5a-5c or the mounting 7, a vacuum-tight joint must be made in order to keep the two sections of waveguide environmentally separate. Vacuum-tight joints, however, are difficult and time-consuming to make, which means that the coupling-out window is relatively difficult and costly to produce. Besides, the more vacuum-tight joints that are made, the higher the probability that one of the joints is not vacuum-tight, thereby rendering the window useless.
Yet another prior art waveguide window is shown in FIG. 5 of the same U.S. Pat. No. 5,051,715. The window shown in FIG. 5 comprises a single dielectric plate 6d having a plurality of parallel recesses 14d-14f carved into the plate 6d. Metal coverings 16a-16c are put over the recesses 14d-14f to form channels through which a coolant may be flowed to remove heat from the plate 6d. In the alternative, metal tubes may be placed within the recesses to provide the desired cooling channels.
The disadvantage of this prior art window is that, in order to successfully carve recesses into the dielectric plate 6d, a relatively-thick plate needs to be used. A thick dielectric plate means that a greater portion of wave power will be lost as electromagnetic waves propagate through the window. This, in turn, causes more heat to be generated.
As discussed above, none of the prior art waveguide windows provide satisfactory results in high power applications. Because of these shortcomings in the prior art, there exists a need for an improved waveguide window which can be used in high power applications and which is easy and economical to produce.